Glass-to-metal seal



June 5, 1951 R. F. DoRAN l 2,555,877

GLASS-TO-METAL SEAL Filed July 20, 1945 Z'f'lo'o Qbo 56o 46o 56o rfwf/z/wf "c INVENTOR ROBERT F. DORAN ATTORNEY Patented June 5, '1951 GLASS-TO-METAL SEAL Robert F. Doran, Floral Park, N. Y., assignor to Sylvania Electric Products Inc., a corporation of Massachusetts Application July 20, 1945, Serial N0. 606,252

Claims.

My invention relates to glass-to-metal seals, such as those which have application to high irequency use and to electron discharge devices employing glass-to-metal seals.

Heretofore, it has been customary in forming glass-to-metal seals between lead-in conductors and glass envelopes, such as for use with electron discharge devices, to use a metal for the lead-in conduction which has a matched coefficient of expansion with the glass. As the leadin conductors are used for electron discharge devices Which have application for the higher frequency currents, it has been found that a conductor which is efficient for lower frequencies may have too high a resistance for higher frequencies. In order to overcome this objection, it has been suggested to employ a sheath of metallic materal over a base material Which may match the glass in expansion behavior or the sheath material may have a sufficiently low yield point to deform when stressed by the surrounding glass body. However, when employing the low conductive sheath around the core, there is the possibility that the seal may break between the sheath and the core material. In order to prevent the rupture between the sheath and the core material, it has been suggested, particularly when copper has been used for the sheath, to braze or Weld the sheath to the core material. Although copper clad materials have the desired low resistance to high frequency current, the use of such material has been limited because of the tendency to excessive oxidation of the copper at the time the seal is made. This is particularly troublesome when the parts are fabricated on high speed production equipment necessitating high temperatures for proper molding of the glass and completion of the seal. The manufacture of seals from silver clad material on production equipment is also critical because the low melting point of the sheath material limits the peak temperature which may be used.

It is, therefore, an object of my invention to provide a glass-to-metal seal which is simple to manufacture, eflicient in operation, and which vwill stand more than the normal amount of rough treatment during further fabrication of the device and during operation of the resulting product.

A further object of my invention is to provide an improved glass-to-metal seal in producing a, stem or header for a radio tube which can withstand high temperatures in air and thus can be used for the fabrication of parts on high speed production machinery.

A further object of my invention is to produce a radio tube with an improved glass-to-metal seal which will maintain the proper seal during subsequent normal use of the radio tube.

A still further object of my invention is to produce a good sealing material which when properly cleaned will have high resistance to subsequent corrosion and consequent low contact resistance when used in a radio tube socket having the usual pressure type of contact.

Further objects of my invention will become apparent from the following description referring to the accompanying drawing, and the features of novelty which characterize my invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.

Referring to the drawing, Fig. l is a sectional view of a high frequency conductor which Ais formed according to my invention; Fig-2 is a section side elevation of a tube header formed of pins of material as illustrated in Fig. 1; Fig. 3 illustrates a modified structure in which a tube of glass is sealed to a tubular conductor; Figs. 4 and 5 illustrate a conductor sealed to glass and are used to illustrate my invention; and Fig. 6 is a graph of the relative expansion characteristics with temperature of soft glass, and a prior iron-nickel-chromium conductor, and a conductor used according to my invention.

In order to provide a conducting material which will seal with soft glass and which has particular application for high frequency, I employ any suitable core material which has an expansion coefficient within the same range as soft glass, such a material being an iron-nickel-chromi-um alloy, such as described in Kingston Patent 2,284,151, but being modified in the manner to be hereinafter described. Such a material has found wide application for use in pins in radio tubes, particularly those having application in the lower frequencies. In order to provide a good conducting path for high frequency currents, however, I provide a coating of material around the core, such coating material being formed of nickel. Referring to Fig. l of the drawing, I have illustrated diagrammatically a core l having a relatively thin coating 2 around it of nickel. It will be understood that other good conductors may be employed provided they have the other desired characteristics which will become apparent as the description proceeds.

I have found, however, that although the nickel coating provides a desirable conductor for high frequencies, the oxide of nickel does not provide a good seal with the glass, and in order to provide an outer surface for the composite conductor which will seal with glass around the nickel coating 2, I provide a coating 3 formed of a material, the oxide of which produces a good bond with the glass, such as chromium. A further and significant advantage in the use of a coating of chromium is that it not only provides an oxide which seals with glass but the underlying chromium rich region. has a very low resistance to ultrahigh frequency currents.

Any suitable desired thickness of the double coating may be employed depending upon the amount of current to be carried and the frequency, and as an example, I have found that a coating of .00075 to .00095 nickel and .00009 to .0001 of chromium is a 4desirable combination to provide over a core .048 inch in diameter.

In order to provide a relatively, thermally stable conducting materiai which may be conveniently sealed to a glass rine to form a stem for an electr'on tube, the conducting material is processed in the manner which will be described below.

The conducting material is first suitably treated to form an alloy between the core material and the coatings, and the material is then treated to produce the desired oxide coating suitable for forming a tenacious bond with the glass. The conducting material 'is rst held at a temperature of 1850 to 2000 F. in an atmosphere of dry hydrogen for a period of between 30 to l5 minutes, depending on whether the low or higher temperature is used. This treatment produces a diffusion process, accomplishes about 80% of the heat work done on the material and is controlled by means of precise time-temperature schedules. As a result of this pre-treatment, the degree of subsequent alloying solid phase reaction occurring o n the stem machine on the previously heatytreated wire is very small. A further advantage bf the controlled alloying cycle is the development of a nickel-chrome alloy sheath having an approximate coeflicient of contraction of 165 x 10"7 between the temperature limits of 400 C. to 25 C. Within the .same temperature limits the c0- eflicient of contraction of the core material is approximately 90 x 10J'. Because of this differential when the coated material is cooled, subsequent to making of the glass-metal seal, radial y compressive stresses develop between the sheath land core material, insuring intimate physical bonding between the two, in addition to the previously described alloying reaction. I have found that the use of my improved pre-treatment before sealing the lead to glass produces a resulting seal which has very consistent and satisfactory characteristics.

After this treatment the conducting material is heated in wet hydrogen to a suitable temperature .to produce .the desired chromium oxide coating. However, because there is a high concentration of chromium on .the surface, I obtain an improved oxide coating suitable for sealing with glass by using a lower temperature and less time than that required heretofore. Thus, I obtain a satisfactory coating of oxide by treating the composite conductor at a temperature of between 1800o to 2300 C. for a time between twenty minutes to ve minutes, depending upon the temperature employed.

It will be understood that a conducting material as described above may be employed with soft glass in any type of configuration, and I have illustrated in Fig. 2 a plurality of pins Il vov which are sealed into a matrix of glass 5, thus producing what is called a header.

In Fig. 3 I have illustrated a tube 6 of material formed according to the description above, and this tube is sealed to a tubular glass body l.

In order to provide a sealed construction such as for use in electronic discharge devices which will stand relatively severe mechanical abuse during further fabrication, operation, and use of the tube, I provide a glass-to-metal seal in which the resulting completed seal is strained in such direction as will most effectively counteract the stresses normally imposed on headers of the type described above. In order to accomplish this, I produce a radial compressive stress between the conductor and the glass matrix at the seal, such stress being indicated as Pr in Fig. 5. Referring to Fig. 4, I have illustrated diagrammatically an enlarged view of the pin 8 which extends through a glass wall 9, such as is found in headers. During the fabrication of and also in the use of the tube, such as by forcing the pins into a socket, the pins usually have applied thereto a resultant force approximately normal to the plane of the seal, such a force being indicated by the arrow lit Such a force will tend to produce a tension in that portion of the seal indicated by the number I0. However, since there has been a pre-stressing of the seal resulting in a radial compression strain, the resultant stress in the vicinity of i0, upon the application of the force F will be less severe in proportion to the relative direction and amount of permanent strain in the body.

In View of the fact that a compressive force between the glass and metal in the construction as illustrated in Figs. 4 and 5 will necessarily produce a circumferential and axial tension component, Pc and Pa, I have found it is desirable to limit the compressive strain in the seal region to the equivalent of a stress of '70 to 300 kilograms per square centimeter. However, these axially and circumferentially tensile stresses do not adversely affect the glass-to-metal seal during opi eration, as in the nal tube structure the principal forces which are applied to the pin are not in these directions.

The technique of obtaining a strained or tempered glass article or composite by means of ydifferential cooling as the glass passes through the strain region of temperature is well known to the art. However, I have found that it is d'iicult to precisely control the amount of residual strain when a diiferential cooling rate technique is employed in a production process. This is especially true when there are metal vparts in the composite or lack of symmetry in the piece, which is the case` when forming a header for an electron discharge device.

In order to obtain compression strain, the base or core material of the metal, which it is desired to seal with glass, is so chosen as to have a lower coefficient of contraction than the glass, especially the contraction from the seal point to room temperature, the seal point being defined as that temperature above which the viscosity of the glass is so low that stress cannot be supported at the glass-metal interface and that temperature below which the viscosity of the glass is sufciently high to prevent dissipation, by means or viscous ow in the glass matrix, of stresses occurring between the glass and metal. the composite is cooled below the seal point, unequal contraction of the metal and glass relative to each other necessarily sets up stress forces which result is strain in the glass and metal, such strain having a peak value at the glassmetal interface. The seal point could more correctly be considered a temperature region, the limits of which are in part dependent upon lthe temperature coefficient of viscosity of the glass under consideration as well as the specific cooling rate of the composite. Practically speaking, the seal lpoint is approximately the temperatureof the intersection with the expansion curve of a line of greater in slope than the average coefficient of expansion between and 300 C.

The differential in the coefficient of contraction between the metal and glass can be accurately measured and controlled so as to obtain the desired amount of residual strain in the con pleted seal when annealed in the normal production type conveyor belt or oven annealer. I have found, for instan-ce, that a 10% lowering of the lowering of the contraction coefficient of the metal below the normal matched coefficient will result in a permanent strain approximately equivalent to a stress of 200 lag/cm?, which I have found to be a very desirable value for the wire size given above. Any suitable type of core material may be employed having the desired expansion characteristics, such as an iron-nickelchromium alloy, and a desirable alloy having the above contraction characteristics is as follows:

Percent Chromium 4 .5 Nickel 42.0 Aluminum .l Silicon .2 Sulphur .015 Phosphorus .015 Manganese .25 Carbon .02 Balance, iron.

Referring to Fig. 5, l show expansion and contraction curves of a lead glass G, metal Ml, which .is a conventional alloy, and metal M2 used according to my invention; the coefficient of contraction of metal M2 being less than that of Mi. and Ml and M2 curves are drawn in such a manner as to cross the glass curve at T! which I indicate as the seal point and which is common to both materials since it is primarily defined by the viscosity-temperature characteristics of the glass. As a composite made from the materials Mi and G is cooled from TI, they contract dimensionally from LI to L2, having the same dimensions at room temperature T3, and therefore, no strain will be found at the seal, although there has occurred temporary transient stresser-1 as indicated by the separation of the respective contraction curves in the temperature range However, with a composito seal made from the materials G and the metal M2 whose coeicient 0f contraction is less than that of MI, it is seen that M2 will contract to length L3, whereas the glass will attempt to contract to the length L2. Therefore, there will occur stresses between M2 and G as a result of the smaller amount of contraction of M2 as indicated by the fact that that (Ll-L2) is greater than (Li -Lii In other words, in the principal stress direction of the composite under consideration, the glass G will be under a radial compression strain because of the inability of metal to contract to L2.

Although I have shown and described particular embodiments of my invention, I do not desire to be limited to the embodiments described, and I intend in the appended claims to cover all modications which do not depart from the spirit and scope my invention.

What I claim is new and desire to secure by Letters Patent of the United States is:

l. A glass-to-metal seal construction between a metallic conductor and soft glass suitable for high frequency, said conductor including a core having an alloy comprising iron, nickel, and chromium having proportions such that the contraction characteristics of the alloy from between the seal point temperature to 25 C. is less than the contraction characteristics of the glass, a relatively thin coating of nickel around said core, and a relatively thin coating of chromium around said nickel so as to provide a relatively low resistance path to high frequency and an oxide to seai with the glass, said core material and coatmaterials being alloyed together so as to minimize the possibility of any breaking of the seal between the core and coating, the region between the glass and metal being under radial compression pressure of about to 300 kilograms per square centimeter so that upon a force being applied to the metal normal to the plane of the seal the possibility of development of tension stresses in the glass is minimized.

2. A glass-to-metal seal construction between a metallic conductor and soft glass suitable for high frequency, said conductor including a core having an alloy comprising iron, nickel, and chromium having proportions such that the contraction characteristics of the alloy from between the seal point temperature to 25 C'. is less than the contraction characteristics of the glass, a relatively thin coating of nickel around said core, and a relatively thin coating of chromium around said nickel so as to provide a relatively low resistance path to high frequency and an oxide to seal with the glass, said core material and coating materials being alloyed together and the contraction property of the coating to be greater than the core so as to minimize the possibility of any breaking of the seal between the core and coat- 3. The method of forming a glass-to-metal seal between a conductor and glass suitable for high frequency including the steps of coating a core material of an iron-nickel-chromium alloy with coatings of nickel and chromium, heating the composite conductor in an atmosphere of dry hydrogen to a temperature of 1850 F. to 2000D F. and holding it there for a period of 15 to 30 minutes to cause an alloying between the core and the coatings so that subsequent heating of the conductor and glass to provide a seal will not substantially affect the characteristics of the conductor, and heating the glass and conductor to the softening point of the glass to provide a seal between the glass and conductor upon cooling.

4. A glass-to-metal seal construction between a metallic conductor and soft glass suitable for high frequency, said conductor including a core of an alloy comprising iron, nickel, and chromium, a coating of nickel adjacent said core, and

a relatively thin coating of chromium adjacent said nickel coating for sealing with said glass, said outer coatings having been alloyed by heat treatment and having an approximate co-eflcient of contraction of x 104.

5. A glass-to-metal seal construction between a metallic conductor and soft glass suitable for high frequency, said conductor including a core of an alloy comprising iron, nickel, and chromium, a coating of nickel adjacent said core, and

a relatively thin coating of chromium adjacent said nickel coating for sealing with said glass, said nickel coating having a thickness of between .00075 to .00095 and said chromium coating having a thickness of .00009 to .0001 with a thickness of core material of about .048 inches said outer coatings having been alloyed by heating treatment and having an approximate coeicient of contraction of 165 x 10-7.

ROBERT F. DORAN.

REFERENCES CITED The following references are of record in the file of this patent:

Number Number UNITED STATES PATENTS Name Date Laise Mar. 9, 1926 Laise Jan. 29, 1935 Scott Oct. 8, 1940 Miller et a1. Nov. 9, 1943 FOREIGN PATENTS Country Date Great Britain July 1, 1926 Great Britain June 15, 1943 

